(1) Field of the Invention
The present invention relates generally to drywall construction, and more particularly to an improved corner bead strip with paper wings and a method for the manufacture thereof.
(2) Background Information
Current building construction utilizes sheets of drywall, commonly referred to as "wallboard", to form the surfaces of interior walls of buildings. Drywall, or wallboard, is formed of sheets of plaster which are sheathed in an outer wrapping of heavy construction paper.
In wallboard construction, the joint between adjacent sheets of wallboard is usually covered by a paper tape extending lengthwise along the joint. The conventional drywall tape is provided in narrow elongated strips of porous paper wound into rolls. The drywall tape is applied to the joints, and then covered with wet plaster or "mud". The plaster is feathered and smoothed along the edges of the tape to conceal the tape edges and form a smooth unmarred surface where the wallboard adjoins.
It is often necessary to cut the wallboard to form a corner, which thereby exposes the plaster contained between the heavy paper sheets. This exposed plaster tends to crumble unless these edges are protected. To finish exterior corners in wallboard construction, metal comer beads are typically installed. Such corner beads are conventionally formed by rollforming from an elongated strip of sheet metal, and provide a rounded nose with two mounting flanges extending at substantially right angles from the opposing sides of the nose. These mounting flanges are often knurled or embossed to provide a rough surface so that the joint compound will adhere when the corner is finished.
The corner bead is installed by securing the mounting flanges along the surface of the drywall panels adjacent to the corner by nails or the like. Wet plaster is then smoothed into place to cover the metal flanges, and edges of the plaster are smoothed and feathered to cover and conceal the metal edges.
A second type of corner bead is referred to as a "tape-on" bead. Tape-on comer beads utilize paper wings to secure a metal comer angle in position, rather than using riails or other fasteners. Wet plaster or joint cement for finishing the corner will normally adhere significantly better to the paper cover strip of tape-on beads, than to the exposed metal of conventional nail-on beads. Nail-on beads are also typically more susceptible to developing crack lines along the outer edges of the flanges, than are tape-on beads. In addition, tape-on beads are more tolerant of dimensional and geometric changes in the underlying construction framing than are nail-on beads with their rigid mechanical attachment to the construction framing.
One of the main problems with prior art tape-on bead is the use of standard joint/drywall tape-on the bead. Such drywall tape is very fibrous, which is good for bond strength, but poor for appearance. During the application of joint cement over the tape, to adhere the corner bead to the drywall, fibers will project and protrude with only minimal contact by the application tools. These fibers will ball up during the course of sanding of the joint cement for the final finish, thereby detracting from the finished appearance of the corner.
One method for improving protection against adverse abrasion of this paper strip is disclosed in U.S. Pat. Nos. 5,613,335 and 5,836,122, both to Rennich et al. These patents disclose a paper bead (tape-on bead) utilizing a paper layer which is uniformly impregnated throughout its thickness with latex or similar strengthening compound with a high wet strength so as to make the paper strip resistant to scuffing and abrasion throughout its thickness. This impregnated stock paper would have a high pick resistance or surface fiber bond, and would effectively inhibit the separation of surface fibers during application on wallboard, thereby providing a good finished appearance in installation. However, the applicants herein have found that paper of this type, which has been impregnated with latex or the like, exhibits poor joint compound bonding properties. Bond Strength Test ASTM C 474 is required by specifications ASTM C 475 and ASTM C 1047 for wallboard accessories manufactured from steel and paper in combination. This ASTM test observes the result of peeling the paper away from a joint compound bond made under controlled conditions.